Only if the prosecutor is a moron.
An attorney may in general only ask someone about something another person said if the person being asked about will be available to the opposing party for cross-examination. For the defense to introduce testimony about things GZ said, it would have to agree to let the prosecution subject GZ to cross-examination by the prosecution. Conversely, the fact that the prosecution asks people what GZ said would mean the defense had the right to call GZ; since the defense already has the right to call GZ, however, the latter point doesn't really matter.
See post 2486.